Production of aluminum



Dec. l, 1959 R. A. I Ewls PRODUCTION oF ALUMINUM Filed NOV. 28, 1955 Fill-3 3.-

Mixure of 73%sodum /3 cryol He, 22%potassum cryolie, 5%olumnu FLE United States Patent() PRODUCTION OF ALUMINUM Robert A. Lewis, Cupertino, Calif., assignor to Kaiser Aluminum & Chemical Corporation, Oakland, Calif., a corporation of Delaware Application November 28, 1955, Serial No. 549,347

23 claims. (ci. 204-61) This invention relates to the production of aluminum by electrolytic reduction of alumina; and, more particularly, it relates to providing an improved electrolyte and electrolyzing system and an improved method of electrolysis for such production of aluminum.

The production of aluminum metalv by electrolysis of alumina dissolved in a fused electrolyte and deposition of aluminum metal at the cathode has long been known. The fused bath employed has consisted essentially of sodium cryolite which is a double salt of sodium fluoride and aluminum uoride and is generally considered by the authorities to have the-formula Na3AlF6 or, expressed in another manner, 3NaFAlF3. This compound has a melting point of about 1000 C. but the melting7 point is decreased by dissolution of alumina in the cryolte. The melting point can also be decreased by addition of another compound, notably sodium iluoride, aluminum fluoride, fluorspar, or sodium chloride, but the addition of another compound or of other compounds to lower the melting point must also take into account the effect of such addition upon the surface tension or the viscosity of the bath content and upon the solubility of alumina in the bath. For all of these, as well as other, reasons, the bath of sodium cryolite is in practice operated at a temperature of about 950 to 970 C. or higher, that is, at atemperature which is not the lowest which could be attained by such additions, if decrease in temperature were the only factor to be considered. However, it is desirable to work at as low a temperature as possible because lower operating temperatures require less addition of heat, greater heat eiciency, reduced losses by volatilization of bath constituents or products and provide other operating advantages.

Potassium compounds have been known to be quite destructive to the `carbon cathode of an electrolytic cell as such cell normally employed in this process and, in fact, it has been recognized that even small amounts of potassium compound must be rigidly excluded from the cell bath constituents in the usual known process, because, even though the original, very small amountV thereof might not be apparently detrimental, potassium, as a compound, will accumulate in the bath and will very soon exert a deleterious effect. It has Vbeen recognized in the art that, the presence inthe cryolite bath of as little as 5% of potassium, calculated as the oxide or as the fluoride, will disrupt the carbon cathode lining of an aluminum reduction cell under the usual conditions of operation where the bath consists essentially of sodium cryolite, the anode is carbon and the cathode is carbon, e.g. as the lining of the bath-containing portion of the cell. Alternatively, it has previously been proposed to employ a potassium-containing bath in a cell wherein the anode is steel coated with nickel and the cathode is molten aluminum in contact with frozen'aluminum leads, but this provides a rather expensive cell construen tion, and has other disadvantages. 4

According to the presentI invention, it has now been found that the cell can be operated at a lower tempera- ICC ture, volatilization of bath componentsreduced, and elliciency of the operation improved by carrying out the reduction of the alumina by a process wherein alumina is dissolved in molten potassium-containing cryolite and electric current is passed through the solution yso obtained from a carbon anode to a negative current conductor which consists essentially of at least one compound chosen from the group consisting ofthe carbides of refractory hard metals andl the borides and nitrides ofthe refractory hard metals, which compounds are resistant to attack by molten aluminum, are Vresistant, to attack by molten cryolite baths, are wetted by molten aluminum and have an electrical conductivity at least greater than graphite.

Figure l' is a vertical sectional view of an electrolytic cell which is suitable for carrying out this invention.

Figure 2 is a plan view of the cell of Figure 1,"taken along line 2 2. i

The molten or fused electrolyte bath employed in the present invention consists essentially of from 5% to V95% of at least one potassium compound chosen from the group consisting of potassium cryolite, potassium chloride and potassium uoride and the remainder sodium cryolite, sodium chloride or fluoride, or lithium chloride or fluoride, lithium cryolite, or any desired mixture of these compounds. The major component of' the bath mixture, i.e. at least 50% thereof, is cryolite, either potassium cryolite, sodium cryolite or lithium cryolite or mixture of these compounds, in order to provide sulicient dissolving power for the alumina to be added. When potassium chloride or uoride, 'or both, is used as freezing point depressant, it is preferred to add not over 30% of either compound when used alone or total, if both are employed, in order tov maintain high alumina solubility. In a bath containing to 90% potassium cryolite, for instance, the freezing point can be advantageously decreased by the addition of 10% to 15% of sodium chloride, lithium chloride, lithium uoride or any desired mixture of these compounds. One bath composition which is quite advantageous, according to this invention, consists essentially of to 70% sodium cryolite and from 10% to 30% potassium cryolte. Another suitable bath composition consists essentially of V80% to 95% sodium cryolite and from 5% to 20% of potassium iluoride or chloride. A further bath composition consists essentially of from 70% to 95% potassium cryolite and from 5% to 30% sodium chloride. It` will be understood that the compositions set forth herein are those of the base baths but that other modifying agents can alsobe, and usually are, admixed therewith. For instance, infoperation the bath will always contain a few percent of alumina, that is, usually and preferably up to the limit of solubility of alumina in the bath, or, in other words, up to a saturated solution of alumina` in the bath. Also there can be added if desired, a small amount, up to 5% of uorspar. Electrolyte baths can be prepared according to the present invention having melting points of less than 950 C., enabling operation of the reduction process at temperatures lower than those required in the processes of the prior art.

The reduction cell or system of the present invention consists essentially, at the time when operation ofthe reduction process is to be carried out, of an anode, preferably of carbon, a fused electrolyte bath consisting essen tially of from 5% to 95% of at least one potassium compound chosen from the group consisting of potassium cryolite, potassium chloride and potassium uoride and the remainder sodium cryolite, sodium chloride or uoride, lithium cryolite, lithium chloride or fluoride, or any desired mixture of these compounds, and a negative current conductor consisting essentially of at least one subor the current conductor can'be a series of plates disposed within the bath in any desired manner and connected in any desired manner to the negative pole of the current `supply. Alternatively, a bath receptacle can be provided with a coating, which can be quite thin or of any thickness desired, of the compound or compounds herein described as suitable, this electric current-conductive coating being connected to a suitable current supply. The compounds useful as conductor materials in the process and cell of this invention include titanium carbide, titanium boridetitanium nitride, zirconium carbide, 'zirconium boride, zirconium nitride, vanadium carbide, va-

nadium boride, vanadium nitride, tantalum carbide, tantalum boride, tantalum nitride, niobium carbide, niobium nitride, and niobium boride, and the boride, carbide and nitride of hafnium, that is, the borides and carbides and nitrides of the metals of subgroups A of Groups IV and V and Series IV, V and VI of the periodic table of elements. If desired, mixtures of these compounds can be employed. The current conductor is a solid mass of the compound or mixture defined, and can be of any fdesired size and shape, for instance, as described above. It has been found that a conductor of the composition described has excellent resistance to attack by a potassiumcontaining electrolytic bath under the conditions of operation of an aluminum reduction cell. The presence of such conductor enables, in an etiicient and inexpensive manner, the operation of an aluminum reduction cell containing in the bath an etfective amount of potassium compound.

The alumina employed as reactant in the method of this invention can be derived from any source. However, the alumina produced by the well known Bayer process is advantageously employed as reactant.

One embodiment of a cell according to the present invention and suitable for carrying out the process of this invention is shown schematically in Figure 1. In this embodiment is a cell-containing shell, suitably of steel, within which is disposed in the usual manner an insulating lining 11 which can be any desired insulating material, such as alumina, bauxite, clay, or aluminum silicate bricks, and in this instance is a packing of granular alumina. Within the insulating lining 11 is disposed bath lining 12, which can beof any desired material, for instance, carbon,

alumina, fused alumina, silicon carbide, silicon nitride, or otherk desired material, and in this instance is high alumina brick. Within bath lining 12 is disposed the electrolyte bath 13 containing a potassium compound as described. At the base or bottom of bath 13 is a pad 14 of aluminum metal. During carrying out of the process of this invention and at the time when aluminum is being produced, bath 13 and metal pad 14 are both in molten or fused state. The anode 15 is of the conventional carbon type and can be either pre-baked or of the self-baking type known to the art, or of any desired sort. The electrolyte bath is covered by a solid crust or cake 16, which consists essentially of frozen bath constituents'and addi- ,tional alumina, in the usual way. As alumina is consumed inthe bath 13, the frozen crust is broken and more alumina fed into the bath in known manner.

disposed within these apertures and extendintopadl4of aluminum metal at the interior, and they also extend exteriorly of shell 10 and are connected to bus bars 19, the latter being connected to the negative pole of a source of electrical current (not shown). A sealing material 20 is packed in around rods 18 within apertures 17 to seal oi the apertures and prevent leakage of the fused bath. Any suitable sealing material can be employed. For example, when the lining 12 is of carbon or silicon carbide, a carbonaceous paste which cokes upon heating can be advantageously employed. However, the bath can also flow into the apertures around the rods and will freeze there and effectively seal these openings. Alternatively, if alumina brick, or grain or grog is employed as cell lining, the rods can be sealed in with calcium aluminate cement, for instance.

Alternatively, the current conductors according to this invention can be plates extending within the bath and connected to metallic connectors leading to the bus bars or other conductors; or the conductor rods can be connected at their-outer ends to metallic connectors leading to bus bars or other conductors.

It is an advantage of the present invention that vthe electrolyte bath has a melting point lower than the electrolytes heretofore useful, and therefore the reaction can be carried out at lower temperatures, for instance, at temperatuers below 950 C. It is a further advantage of the present invention that the cell lining in contact with the electrolyte bath can be carbon and that the carbon will not be disintegrated because the path of the current is through the current conductors as defined herein. Carbon is disrupted and disintegrated when it acts as a current conductor in contact with an appreciable amount of potassium compound but the present cell system and process avoid this disadvantage.

In an example of the mode of carrying out the process of thepresent invention. in a reduction cell such as that shown in Figures l and 2, cell bath 13 consists essentially of a mixture of 73% sodium cryolite, 22% potassium cryolite (3KF-AlFg) and 5% alumina and the furnace temperature is maintained at 940 C. while electric current is passed through the bath from the anode to the cathode, the current density being maintained at an average value of S amperes per square centimeter. The conductor rods 18 are composed of zirconium boride and lining 12 is of'high alumina brick. In starting up the furnace, fused electrolyte is poured in from a melting furnace and sutiicient molten aluminum is added to form a pad 14 at the bottom of the electrolyte. yAlternatively no molten aluminum is added because the negative conductors employed in the presentfcell and method exhibited excellent electrical conductivity and are wetted bv the deposited aluminum, so that provision of an aluminum pad priorto'or at the time of starting the reaction is notessential. The rate of production of Aaluminumris'quite rapid, and it formsa pool or pad at the bottom of the bath, vfrom which molten aluminum is withdrawn at intervals. for instance by siphoning on in the known manner. The aluminum pad which forms during operation of the cell is the cathode. and the current passes therefrom bv way of the current conductors provided according to this invention.

The following comparative test demonstrate clearly the stability of the cell system of the present invention and the production of aluminum metal by the present process. Each of these tests is carried out by placing in a graphite Crucible, which is suitably connected to a source of electrical current and which serves as anode, 300 grams of molten bath compositions as follows:

(A) Reduction bath electrolyte as used in prior art commercial production of aluminum, and containing 88% sodium cryolite (NaaAlF), 8% uorspar CaF2) and 4% alumina (A1203).

(B) Reduction bath electrolyte according to the present invention and containing 73% sodium cryolite `(NaaAlF), 22% -potassium cryolite ('KgAlFG) land 5% alumina (Al-203), which corresponds to about 77% sodium cryolite and about 23% potassium cryolite, based on the amounts of these two components alone.

The negative current leads or cathodes employed in the various tests are identified in Table I below, the carbon cathodes being prepared from arc-welding electrodes, and the other cathodes being zirconium boride rods and titanium carbide rods. Each current lead is disposed vertically over the crucible and immersed about 1.5 inch in the molten bath, and electric current is turned on, being applied at a cathodic current density of 5 amperes per square inch. Bath A, which corresponds to commercial practice of the known art, is maintained at 970 C.; and baths B, at 940 C. Table I sets forth the eiect of the electrolyte upon the respective current leads, in each test.

current through said bath from an anode to a negative current`conductor consisting essentially of at least one substance chosen from the group consisting of the borides, carbides and nitrides of titanium, zirconium, vanadium, tantalum, niobium and hafnium.

2. Process as in claim 1 wherein said negative current conductor consists essentially of titanium carbide.

3. Process as in claim l wherein said negative current conductor consists essentially of zirconium boride.

4. Process as in claim l wherein said bath consists essentially of from 70% to 90% sodium cryolite and from 10% to 30% potassium cryolite.

5. Process as in claim 1 wherein said negative current conductor consists essentially of titanium boride.

6. Process as in claim l wherein said negative current conductor consists essentially of zirconium carbide.

Table l Test No. Bath Temp., Length of Cathode Observations C. Run

1 A (Control). 970 3% hrs Carbon.-- Some cracking of cathode. Balls of aluminum found at the bottom of the graphite crucible. A few balls adhering to cathode.

2 B 940 ..-.-do ---do Severe cracking of cathode. No aluminum found ln crucible or on cathode.

3-. B 940 5 minutes-.. ---do Cathodes badly eroded. Run stopped.

4. B. 940 2hrs -do.... Portion of cathode immersed in melt is 'completely disintegrated.

5 B 940 3% hrs ZrB, Considerable aluminum adhering to cathode. So'ne Al crept along cathode above melt level. After grinding off aluminum, there is no evidence of cracking or appreciable attack.

6 B 940 do TiC Considerable aluminum ad heres to cathode. after grinding ofi no evidence oi cracking or appreciable attack.

It can be seen that in the cell system and by the process of the present invention aluminum metal is produced at lower temperature and that the cathodes remain in good condition. On the other hand, in Tests Nos. 2, 3 and 4, not only is the carbon rod badly attacked, but no aluminum metal is recovered, in each case.

Other structures than those shown in Figs. 1 and 2 can be employed in arranging the cell system of the present invention. For instance, the current conductor rods can pass upwardly through the bottom of the cell lining, and extend substantially vertically into the bottom portion of the bath. Alternatively, the current conductor can be a plate or plates disposed in the bath at an angle to the horizontal axis of the bath, the plate or plates being suitably connected to a bus bar or other electrical conductor, in the manner known to the art.

In this specification and claims percentages are by Weight unless otherwise indicated. The terms potassium cryolite and sodium cryolite refer respectively to composition of the general formulae, BKF-AlFa and SNaF-AIF3, but it will be understood that an excess of the alkali metal fluoride or the aluminum fluoride can be present as desired. Although alumina is a reactant in this system and when added as such dissolves in the fused bath, alumina brick or grog can be used as cell lining because a protective layer of bath freezes on the side walls thereof and prevents further substantial dissolution, the aluminum pad protecting the bottom of the cell.

Having now described the invention, what is claimed 1s:

1. In a process for the electrolytic production of aluminum the steps which comprise dissolving alumina in a fused bath of electrolyte consisting essentially of from 5% to 95% of at least one potassium compound chosen from the group consisting of potassium cryolite, potassium chloride and potassium uoride, and from 95% to 5% of at least one substance chosen from the group consisting of sodium cryolite, sodium chloride, sodium lluoride, lithium cryolite, lithium chloride and lithium uoride, and containing at least 50% of cryolite, and passing an electric 7. In a process for the electrolytic production of aluminum the steps which comprise dissolving alumina in a fused electrolyte bath consisting essentially of about 77% sodium cryolite and 23% potassium cryolite and passing an electric current through said bath from a carbon anode to a negative current conductor consisting essentially of zirconium boride.

8. ln a process for the electrolytic production of aluminum the steps which comprise dissolving alumina in a fused electrolyte bath consisting essentially of about 77% sodium cryolite and 23% potassium cryolite and passing an electric current through said bath from a carbon anode to a negative current conductor consisting essentially of titanium carbide.

9. In a process for the electrolytic production of aluminum the steps of preparing a cell bath consisting essentially of 73% sodium cryolite, 22% potassium cryolite and 5% alumina, maintaining the temperature at 940 C., and passing electric current through said bath from a carbon anode to a negative current conductor consisting essentially of zirconium boride.

l0. In a process for the electrolytic production of aluminum the steps which comprise dissolving alumina in a fused electrolyte bath consisting essentially of from 5% to of at least one substance chosen from the group consisting of potassium cryolite, potassium chloride and potassium fluoride and from 95% to 5% of at least one substance chosen from the group consisting of sodium cryolite, sodium chloride, sodium fluoride, lithium cryolite, lithium chloride and lithium iluoride, and containing at least 50% cryolite, and passing an electric current through said bath and a molten aluminum cathode to a negative current conductor consisting essentially of at least one substance chosen from the group consisting of the borides, carbides and nitrides of titanium, zirconium, vanadium, tantalum, niobium and hafnium.

11. Process as in claim 10 wherein said negative current conductor consists essentially of titanium carbide.

l2. Process as in claim l0 wherein said negative current conductor consists essentially of titanium boride.

13. ;Process A'as in claim ,10 wherein said negative current conductor consists essentially of zirconium boride.

14. .Process as in claim 10 wherein said negative current conductor consists essentially of zirconium carbide.

15. Process as in claim 10 wherein said fused bath consists essentially of from 70% to 90% sodium cryolite and from 10% to 30% potassium cryolite,

,16. .Process asin claim 15 wherein the negative current conductor'is zirconium boride.

17. Process as in claim 15 wherein the negative current conductor is titanium carbide.

18. In a process for the vrelectrolytic production of aluminum, the steps which comprise dissolving alumina in a fused bath of electrolyte consisting essentially of from to.95% of'at least onepotassium compound chosen from the group consisting of potassium cryolite, potassium chloride and potassium fluoride, and from 95% to 5% of at least one substance chosen from the group consisting of sodium cryolite, sodium chloride, ,Sodium uoride lithium cryolite, lithium chloride and lithium fluoride, and containing at least 50% of cryolite, and passing an electric current through said bathtrom an anode to at least one negative current conductorfconsisting essentially of at least one substance chosen from the group consisting of the borides, carbides and nitrides of titanium, zirconium, vanadium, tantalum, niobium and lhafnium the temperature of said bath beingfbelowabout 950 C.

19. A process as in claim 18 whereinsaid negative current conductor consists essentially of titanium boride.

20. Process as in claim 18 wherein said negativecurrent conductor consists essentiallyfof titanium carbide.

'21. Process as in claim 18 wherein said negative current conductor consists essentially of zirconiumboride.

22. Process as in claim 18 wherein said negative current conductor consists essentially of zirconium carbide.

23. Ina process for the eleetrolytic production of aluminum, the steps which comprise dissolving alumina in a fused bath of electrolyte consisting essentially of from 5% to 95% of at least one potassium compound chosen from the group consisting of potassium cryolite, potassium chloride and potassium uoride and from`95% to 5% oi atV least one substance chosen from the group consisting of sodium cryolite, sodium chloride, sodium uoride, lithium cryolite, lithium chloride and lithium fluoride, and containing at least of cryolite, and passing an electric current through said bath from at least one anode to a. molten aluminum cathode and then to at least one negative current conductor in contact with said molten aluminum cathode, :said negative current conductor consisting essentially of atleast one substance chosenfromthe group consisting of the borides and carbides of titanium and zirconium, said fused bath of electrolyte being at a temperature below about 950 C.

`References Cited in the iile of this patent UNITED STATES PATENTS 400,664 Hall Apr. '2, 1889 FOREIGN PATENTS y1,064,743 France Dec. 3,0, 1953 UNITED STATES PATENT oEETCE CERTIFICATE 0F @ORBECTIN Patent Noe 2915q442 December 1, 1959 Robert A., Lewis It. is hereby certified that error appears in June printed specification of the' above numbered patent requiring correction and tha-b the said Letters Patent should read as corrected below.

Column 3 line 9*7 for "or" second ooculrlfenoel read of w; column 4 line 62,? for "test" read tests 11ne 71x7 for "(23172) read e (CHFQ) Signed and sealed this 31st day of January 1961B (SEAL) Attest:

KARL Hf AXLINE ROBERT c. WAT-Solv Attesting Ocer Commissioner of Patents 

1. IN A PROCESS FOR THE ELECTROLYTIC PRODUCTION OF ALUMINUM THE STEPS WHICH COMPRISE DISSOLVING ALUMINA IN A FUSED BATH OF ELECTROLYTE CONSISTING ESSENTIALLY OF FROM 5% TO 95% OF AT LEAST ONE POTASSIUM COMPOUND CHOSEN FROM THE GROUP CONSISTING OF POTASSIUM CRYOLITE POTASIUM CHLORIDE AND POTASSIUM FLURIDE, AND FROM 95% TO 5% OF AT LEAST ONE SUBSTANCE CHOSEN FROM THE GROUP CONSISTING OF SODIUM CRYOLITE, SODIUM CHLORIDE, SODIUM FLUORIDE, LITHIUM CRYOLITE, LITHIUM CHLORIDE AND LITHIUM FLURIDE, AND CONTAINING AT LEAST 50% OF CRYOLITE AND PASSING AN ELECTRIC CURRENT THROUGH SAID BATH FROM AN ANODE TO A NEGATIVE CURRENT CONDUCTOR CONSISTING ESSENTIALLY OF AT LEAST ONE SUBSTANCE CHOSEN FROM THE GROUP CONSISTING OF THE BORIDES, CARBIDES AND NITRIDES OF TITANIUM, ZIRCONIUM, VANADIUM, TANTALUM, NIOBIUM AND HAFNIUM. 